The proposed research program involves applications of magnetic resonsance spectroscopy to investigate interactions between a lipolytic enzyme and aggregated phospholipids and between metabolic enzymes and their substrate molecules. The objective of the research is to determine structures of the complexes formed between enzymes and substrates and between proteins and aggregated phospholipids in order to elucidate the mechanism of the respective catalytic reactions and factors which control or regulate the metabolic activities. The lipolytic enzyme under investigation is phospholipase A2 from the digestive secretion of mammalian pancreas. This enzyme catalyzes hydrolysis of fatty acid ester linkages of acylphosphoglycerides which are ubiquitous constituents of biological membranes. The enzyme is activated upon binding to an appropriate lipid-water interface. The detailed mechanism for this activation and the structure of the protein-phospholipid complexes are not understood. Magnetic resonance spectroscopy and precision infrared difference spectroscopy are used to obtain detailed structural information on enzyme-substrate complexes with enzymes of the kinase and ligase classifications. The mechanistic roles of inorganic cations in the catalytic activities of these enzymes are under investigation.